1. i. What is meant by capacitance grading of a cable ? (M ay 11, Nov 10) ii. Derive expressions for capacitance of and maxi mum potential gradient in two ( or more )
dielectric of a graded cable in terms of a dielectric constants and radius of core and overall radius etc.
2. i. Show that in a capacitance graded cable the position of different layers is decided by the product r g W here is the relative permittivity of the diele ctric and g is the dielectric strength (potential gradient) and that for a cable with overall radius R having (say) 3 dielectrics with all dielectrics working at the same maximum potential gradient r 1r = r 2r1
= r 3r2 where r, r1, r2 are the radii of conductor, inner and middle dielectric respectively.
ii. Write a short note on single core cable with a neat diagram? (M ay 11, Nov 10)
3. Show that in a three -core ( belted type ) cable the neutral capacitance of each conductor Cn is equal to where Ce + 3 Cc are the capacitance of each conductor to sheath and to each other respectively. Explain ho w these capacitance can be measured experimentally.
(M ay 11)
4. A 1 -core lead sheath cable joint has a conductor of diameter 8 mm and two la yers of different insulating materials each 8 mm thick. The relative permittivity are r 1 =3(inner)
r2 =2.5(outer). Calculate the potential gradient at surface of the conductor when are p.d.
Between the conductor and lead sheath is 50 kv. (Nov 10)
5. i. Write a short note on pressure cables.
ii. Calculate the insulation resistance for a 5 km length of a 1 -core cable. Resistance o f insulation (impregnated paper) is 5*1014 ohm -c m, insulation thickness is 1 cm and radius
of conductor is 1.25 cm. (Nov 10)
6. i. Deduce the expression for capacitance of a single core cable ?
ii. Write a short note on oil -filled cables ? (M ay 10)
7. Describe the various methods of laying underground cables. W hat are the relative advantages and disadvantages of each method? C ompare the merits and demerits of
underground cables versus overhead system. (M ay 10)
8. Compare the merits and demerits of underground cables versus overhead system T he insulation resistance for a 1 -core cable is 495micro ohms per km. If the core diamet er is 3.5 cm and receptivity of insulation is4.5 *1014 ohm -cm. Find the insulation thickness.
(M ay 10)
9. i. State the classification of cables (according to voltage) and discuss their general construction
ii. Explain carefully the constructional difference between and application of a. deleted
b. screened (H type) c. S.L and
d. H.S.L types of cables.
iii. Write a short note on pressure cables. (M ay 10)
10. i. Find the radius of the intersheaths and t he potential at which it must be maintained for a single core metal sheathed cable such that the overall radius of the cable is minimum. The operating voltage of the cable is V, its core and intersheath diameters are d and D respectively. Sho w also that D= 3.76 V/gm a x.
ii. A 3 -phase, 6 km long belted cable has capacitance per km of 0.5 F bet ween the two cores with the third core connected to the lead sheath. Calculate the charging current taken by of this cable when connected to a 3 -phase, 50 Hz, 11 kV supp ly. (M ay 10) 11. i. Explain the intersheath grading of cable.
ii. A cable has intersheath grading that satisfies the equation, R/r1 = r1/r =. The core and cable radii are r =1.2 cm and R = 3 c m. Deter mine the location of the intersheath and also calculate the ratio of maximum electric field strengths with and without intersheath
grading. (Nov 09)
12. i. In which method a homogeneous dielectric is used for cable grading? Prove that dielectric stress in each layer is the same.
ii. A 3 -phase,66 kV, single core cable of conductor diameter 2.5 cm and lead sheath of
between the core and lead sheath. If the maximum stress in layers is the same. Find the voltages on the inter shea ths.
(Nov 09) 13. i. Describe the general construction of an underground cable with a neat sketch.
ii. A single core lead sheathed cable has the core diameter of 1.2 cm and is graded by using two dielectrics of relative permittivity 3.5 (inner) and 3.0 (outer). The thickness of each being 1 cm, system voltage is 66 kV, 3phase. Determine the potential gradient at the
surface of the conductor and at the other points. (Nov 09)
14. i. Describe briefly some commonly used insulating materials for cables.
ii. A 12.5 kV single -core cable has an outside diameter of 8 cm. Determine the radius of the core and the electric field strength that must be withstand by the insulating material in the
most economical (optimal -ratio) configuration. (Nov 09)
15. i. Derive the formula for insulation resistance of a cable.
ii. Determine the economical core diameter of a single core cable working on 22 kV, single phase system. The maximum permissible stress in the dielectric is not to exceed 33 kV/cm.
(Nov 08)
16. i. Derive a formula for capacitance of a single core cable.
ii. Determine the economical core diameter of a single core cable working on 210 kV, 3 -phase system. The maximum permissible stress in the dielectric is not to exceed 230
kV/cm. (Nov 08, 07)
17. i. Derive a relation between the conductor radius and inside sheath radius of a single core cable so that the electric stress of the conductor surface may be minimum.
ii. A cable has been insulated with two insulating materials having permittivity of 6 and 4 respectively. The inner and outer dia meter of a cable is 3 cms and 7 cms. If the dielectric stress is 50 kV/cm and 30 kV/cm, calculate the radial thickness of each insulating layer
and the safe working voltage of the cable. (Nov 08)
18. i. What are the causes of cable breakdown? W hat are voids? How are they formed? Why do voids lead to cable failure?
ii. A single core lead sheathed cable is graded by using t wo dielectrics of relative permittivity 3.8 (inner) and 2.8 (outer), the thickness of each being 0.75cm. T he core diameter is 1 cm; syste m voltage is 66 kV, 3 -phase. Deter mine the maxi mum stress in two
dielectrics. (Nov 08)
19. i. Show that for the same dimensions of a cable with an intersheath can with stand a working voltage of 33% higher than a non -inter sheath cable. Assume same homogeneous dielectric and most economical designs for both cables.
ii. A 3 -phase, single core 66 kV cable has a conductor diameter of 3 cm and a sheath of inside diameter 6 cm. If two intersheaths are introduced in such a way t hat the stress varies bet ween the same maximum and minimum in the three layers. Find
a. Positions of intersheaths b. voltage on the intersheaths
c. Maximum and mini mum stress. (Feb 08)
20 * i. Compare the merits and demerits of underground system and overhead system.
ii. Determine the thickness of insulation and operating voltage of a single core cable if the maximum and minimum stress in the dielectric is 38 kV/cm (r.m.s) and 12 kV/cm (r. m.s) respectively and the diameter of core is 3 cm. (Nov12, Feb 08) 21. i. Discuss the methods of grading of cables. W hy are they not used generally?
ii. A three -phase, single core, lead covered cable has radius of core 0.5 cm and internal diameter of sheath 6 cm. Its 3 insulating materials A, B, and C have relativ e permittivit y
of 4, 4, and 2.5 with maximum permissible stress of 50, 40, and 30 kV/cm respectively.
Find the operating voltage of the cable. (Feb 08)
22. i. What do you understand by grading of cable? Explain why grading is more of theoretical interest than practical? What is the modern practice adopted to avoid grading?
ii. Determine the maximum and minimum stress in the insulation of a 33 kV single core cable which has a core diameter of 1.5 cm and a sheath of inside diameter 5 cm. (Feb 08, Nov 07)
23. A 66kV concentric cables with t wo inter sheaths has a core diameter 1.8 cm. Di -electric material 3.5 mm thick constitutes the three zones of insulation. Determine the maximum stress in each of the three layers if 20kV is maintained across each of the i nner two layers.
(Feb 08, M ay 05)
24. i. Derive the formula fpr insulation resistance of a UG cable.
ii. In a coaxial cable the conductor diameter is 10 mm and the inner shath diameter is 50mm.
There are two layers of insulation, the inner layer of dielectric constant 4 and a maximu m working gradient of 6kV/mm has a radial thickness of 4.6 mm; the outer layer has dielectric constant 2.5 and maxi mum voltage gradient 5kV/mm. Calculate the maximum
working voltage for the cable. (Feb 08, Nov 07)
25. i. A single core cable has an inner diameter of 5cms and a core diameter of 1.5cm. Its paper dielectric has a working maximum dielectric stress of 60kV/cm. Calculate the maximum permissible line voltage when such cables are used on a 3 -phase power system.
ii. A 66kV concentric cable with two inter sheaths has a core diameter 1.8 cm. Dielectric material 3.5mm thick constitutes the three zones of insulation. Determine the maximum stress in each of the three layers if 20kV is maintained across each of the i nner two layers.
(Feb 08, Nov 07, 06, M ar 06)
26. A single core cable has an inner diameter of 5cms and a core diameter of 1.5cm. Its paper dielectric has a working maximum dielectric stress of 60 kV/cm. Calculate the maximu m permissible line voltage wh en such cables are used on a 3 -phase po wer system. (Feb 08, Nov 04)
27. i. Derive the formula for dielectric stress in an UG cable.
ii. Single -core, lead covered cable is to be designed for 66kV to earth. Its conductor radius is 10mm and its three insul ating materials A,B and C have relative permittivities of 5,4 and 3 respectively and corresponding maximum permissible stresses of 3.8, 2.6 and 2.0 kV/mm (rms) respectively. Find the minimum diameter of the lead sheath. (Feb 08, Nov 04) 28 * i. Show that i n a three core belted cable the neutral capacitance to each conductor Cu is
equal to CS+3CC where CS and CC are capacitance of each conductor to sheath and to each other respectively.
ii. A single core 11 kV, 50Hz , 5 km long cable has a core diameter of 1.5 cm and diameter of under sheath 3.0 cm. The relative permittivity of the insulating material is 2.5. The power factor on open circuit is 0.04. Determine
a. the capacitance of the cab le b. charging per conductor c. dielectric loss
d. The equivalent insulation resistance. ( Nov 13, Nov 07)
29. i. Derive a formula for calculating the current rating of a cable.
ii. Single core, lead covered cable is to be designed for 66 kV to ear th. Its conductor radius is
30. i. Explain briefly intershedts grading of an UG cable.
ii. A circuit, 10km long, consists of three single core cables is connected to a 33kV, 3 -phase, 50 Hz supply. The core of each cable is 10 mm diameter and the dielectric of relative permittivity 2.25, has a radial thickness of 6mm. If the total dielectric loss in the circuit is 10.5 kW and the capacitance to neutral of each cable is 2.55 µF, determine the loss angle
of the dielectric. (Nov 07)
31. i. Give the list of various types of UG cables.
ii. Determine the operating voltage of a single core cable of diameter 2 cm and having three insulating material of permittivites 5, 4, 3. The overall diameter of the cable is 5cm and the maximum working stress is 40kV/cm. Compare the operating voltage with the voltage if the cable were not graded and the material
with same working stress was used. (Nov 06)
32. i. Give merits and demerits of UG cables.
ii. The test results for 1km of a 3-phase metal sheathed belted cable gave a measured capacitance of 0.7µF between one conductor and the other two conductors bunched together with the earth sheath and 1.2µF measured between the three bunched conductor and the sheath. Find
a. the capacitance between any pair of conductors, the sheath being isolated
b. the charging current when the cable is connected to 11kV, 50Hz supply (Nov 06) 33. i. What do you mean by grading of cables? Explain briefly different types of grading of cables
ii. A conductor of 1 cm diameter passes centrally through a porcelain cylinder of internal diameter 2cms and external diaeter 7cms. The cylinder is surrounded by a tightly fitting metal sheath. The permittivity of porcelain is 5 and the peak voltage gradient in air must not exceed 34kV/cm. Determine the maximum safe
working voltage. (Nov 06)
34. What are the limitations of solid type cables? How are these overcome in pressure Cables?
(June 06, 03, Nov 03)
35. i. What is the relation between the conductor diameter and breakdown potential of a cable while voltage of the cable and its overall diameter are fixed? Derive the same.
ii. The capacitance per kilometer of a 3 phase belted cable is 0.25µF between the t wo cores with the third core connected to the lead sheath. Calculate the charging current taken by five kilometers of this cable when connected to a 3 phase, 50Hz, 11kV supply.
(M ar 06)
36. A single core lead covered cable is to be designed for 66kV to earth. Its conductor radius is 0.5cm and its three insulating material A,B and C have relative permittivities 4, 4.5 and 2.5 with maximum permissible stresses of 50,40 and 30 kV/cm respectively. Find the
minimum internal diameter of the lead sheath.
(M ar 06)
37. Determine the economical size of a single core cable working on 220kV, 3-phase system. The maximum permissible stress in the dielectric is not to exceed 250kV/cm. (M ar 06) 38. i. Explain with neat sketch, the general construction of a 3 -conductor cable.
ii. Test results on a 25 km, 2 -core single -phase metal -sheathed cable are as follows.
a. Capacitance per km between the cores bunched and the sheath is 100 µF.
b. With the sheath insulated, capacitance per km between the cores is 0.5µF.
c. Calculate the core -to -core capacitance assuming equal capacitance between each core and the sheath.
d. Also estimate the total charging current required for the cable when connected to 1 kw,
50 Hz supply mains. (M ar 06)
39. A single core cable has an inner diameter of 5cms and a core diameter of 1.5cm. Its paper dielectric has a working maximum dielectric stress of 60 kV/cm. Calculate the maximu m permissible line voltage when such ca bles are used on a 3 -phase po wer system. (Nov 05, 04)
40. i. What are the different types of losses taking place in Cables? Give brief account of them.
ii. A single -core cable 5km long has an insulation resistance of 0.4 MW. The core diameter is 20 mm and the diameter of the cable over the insulation is 50mm. Calculate the resistivit y
of the insulating material. Derive the formula used. (Nov 05)
41. With a neat diagram, show the various parts of a high voltage single core cable.
(Nov05, 03)
42. i. Explain the factors which decides the rating of a cable.
ii. Classify the under ground cables according to various parameters. (Nov 04) 43. Single -core, lead sheathed cable joint has a conductor of 10mm diameter and two layers o f
different insulating m aterials, each 10mm thick. The relative permittivities are 3(inner) and 2.5(outer). Calculate the potential gradient at the surface of the conductor when the potential difference between the conductor and the lead sheathing is 60kV. (Nov 04) 44. Find the diametral dimensions for the I -core, metal -sheathed cable giving the greatest
economy of insulating material for a working voltage of 85 kv, if a dielectric stress of 60 kv per cm can be allo wed.
(Nov 03)
45. i. A single core cable has conductor diamete r of 40mm and the internal diameter of the lead sheath of 90mm. The cable is provided with two different insulating materials having relative permittivity 4.5 (inner), and 3.5 (outer) respectively. The corresponding maximum permissible electric stresses ar e 4.5 and 3.5 kv/mm.
ii. Determine the radial thickness of the insulating materials required to confirm with the above specifications. Also find the safe operating voltage of the cable. (Nov 03)
46. Derive the expression for the insulation resistance o f a single core cable. A 11kv, 50Hz, single -phase cable has a diameter of 10mm and an internal sheath radius of 15mm. If the dielectric has a relative permittivity of 24, determine for a 2.5 km length cable (i) the
capacitance (ii) the charging current. (Nov 03)
47. Derive an expression for the capacitance of a single core cable. (Nov 03, 02) 48. The insulation resistance of a single core cable is 495M Ohms/km. If the core diameter is
2.5cm and resistivity of insulation is 4.5x1014 Ohm -c m. Find the insulation thickness.
(Nov 03)
49. i. What are all insulating materials used in UG cables? Explain in detail about the different kinds of insulating materials
ii. Determine the economical size of a single core cable working on 220 KV, 3 phase syste m, the maximum permissible stress in dielectric is not to exceed 250kV/cm (Jun 03) 50. i. What is the relation between the conductor diameter and break down potential of a cable
while voltage of the cable and its overall diameter are fixed? Prove the same.
ii. The capacitance per Km of a 3 phase belted cable is 0.25 micro farads between the t wo core with the third core connected to the lead sheath. Calculate the charging current take n by five Km of the cable when connected to a 3 phase, 50 Hz supply. (Jun 03)
ii. A 3 -core, 11 kV Cable supplies a load of 1500 KW at 0.85 pf lag for 280 days in a year at an average of 9 hours per day. The capital cost per KM of the cable is Rs 8000a+20000.
The resistance per Km of a cable of cross sectional area of 1 sq. cm is 0.173 ohms. If the energy loss cost per unit is 2 paisa and the rate of interest and depreciation is 12%.Calculate the most economical current density and dia of conductor. (Jun 03) 52. A single core cable is to be designed for 66kV to earth. Its conductor radius is 0.5 cm and
its three insulating materials A, B and C have relative per mittivity of 4, 4.5 and 2.5 with max. Permissible stresses of 50, 40 and 30 kV/cm respectively. Find the min internal diameter of the lead sheath.
(Jun 03)
53. i. Describe with neat sketch the construction of 3 -core belted cable. Discuss the limitation s of such a cable.
ii. A 66 kV single core cable has a conductor diameter of 2.5cm and a sheath of inside diameter 6cm. Calculate the max stress. It is desired to reduce this stress by using two inter sheaths. Determine their best positions, the maximum stress and voltage on each.
(Nov 02)
54. i. Prove that for a concentric cable of given dimension and given maximum potential gradient in the dielectric, the maximum permissible voltage between the core and the sheath is independent of the permittivity of the insulating material.
ii. A single core 66kV cable working on a 3 phase system has a conducto r diameter of 2 and a sheath inside diameter 5.5 cm. If the two inter sheath are introduced in such a wa y that the varies bet ween the same maximum and minimum in the three layer find:
a. Positions of the inter sheath b. Voltage on the inter sheath
c. Maximum and mini mum sheath (Nov 02)
55. i. Derive the expression for capacitance in the 3 phase cable?
ii. cable has insulated with two insulating materials having permittivity of 6 and 4 respectively. The inner and outer dia of the cable are 3 and 7 cm s. The dielectric stresses of 58 KV/c m and 28 KV/cm. Calculate the radial thickness of each insulating material and the safe working voltage of the cable
(Nov 02)
56. i. Find the expression for capacitance and dielectric stress of a single core cable?
ii. Derive expression for power in 3 phase in a 3 phase circuit using symmetrical components? (Nov 02)
57. The L/C ratio for 132 KV and 400 KV lines are tropically 160 x 103 and 62.5 x 103 respectively. That the natural 3 -phase loading for the two line s? (GATE 01) 58. A 6.6kV, 50 hz single core lead -sheath cable has the following data:
Conductor diameter: 1.5 cm, length : 4 km Internal diameter of the sheath : 3 cm resistivity of insulation : 1.3x101 2 ohm-m Relative permittivity of insulation : 3.5 Calculate
i. insulation resistance ii. the capacitance
iii. Themaximum electric stress in the insulation (GATE 99) 59. An overhead line is having a surge impedence of 400 ohm is connected in series with an
underground cable having a surge impedence of 100 ohm. If a surge of 50KV travels from the line end to wards the line cable junctions, the value of trasmitted voltage at the
junction is ? (GATE 99)
60. The cable has the following charecteristics L=0.201 micro H/m and C=1 96.2pF/m. The velocity of the wave propagation through the cable is (GATE 98) 61. Why shunt capacitors are preferred over series capacitors for improvement of po wer factor
in distribution systems? (IES 03)
62. T wo long overhead transmission lines A and B having surge impedances of 400 ohms and 420 ohms respectively, are connected by a short underground cable C of surge impedance 50 ohms. A rectangular surge of magnitude 100 kV and of infinite length travels along A towards the cable C. Find out the su rge voltage which is transmitted into the cable at the
62. T wo long overhead transmission lines A and B having surge impedances of 400 ohms and 420 ohms respectively, are connected by a short underground cable C of surge impedance 50 ohms. A rectangular surge of magnitude 100 kV and of infinite length travels along A towards the cable C. Find out the su rge voltage which is transmitted into the cable at the